This invention relates to metal plating on aluminum. More particularly, this invention relates to an improved process to provide an adherent coating of metal on aluminum which may provide the basis for subsequent electroplating of other metals thereon.
Natural oxides on aluminum inhibit the direct electroplating thereon of other metals. To overcome this difficulty, it has become a standard practice to immerse the aluminum in an alkaline-zincate solution. This treatment results in dissolution of the aluminum oxide surface and a deposition of zinc thereon which in turn apparently prevents any further formation of aluminum oxide on the surface. Korpiun U.S. Pat. No. 2,142,564 describes such a typical process while teaching the addition of copper salts as well to such a zincate bath.
Usually, such zinc deposition is followed by electroplating of copper and/or nickel thereon to provide a proper base layer for the subsequent deposition of a final layer such as chromium. Patrie U.S. Pat. No. 2,745,799, for example, teaches the deposition of zinc from an alkalicyanide bath followed by the plating of nickel thereon.
Unfortunately, however, while the treatment with zinc dissolves the undesired aluminum oxide surfaces, subsequent exposure of the plated aluminum surface to a corrosive environment can result in an undermining type of corrosion due to zinc being anodic to both the plated metal top coat and the aluminum substrate.
This problem has been previously recognized and others have attempted to remove the zinc coating on the aluminum prior to commencing subsequent plating operations. For example, Passal U.S. Pat. No. 2,662,054 uses an alkali metal zincate solution which he then dissolves by immersing the coated aluminum article in a "known chromic acid-catalyst radical chromium plating bath". Passal states that the aluminum article is connected in an electric circuit as a cathode but the circuit connection is not necessarily made at the time the zinc and copper coated aluminum article is first immersed in the solution. He does, however, state that the plating should commence within 15 seconds after the article has been immersed in the chromium plating electrolyte. Forestek U.S. Pat. No. 2,739,932 also applies a zinc coating to the aluminum to replace the oxide film and prevent reoxidation. He then subsequently removes this zinc coating by immersing the coated aluminum member in a concentrated electrolytic solution including chromic acid and sulfuric acid which is also used as the plating bath for plating chromium onto the aluminum article.
While these processes for dissolving the zinc layer have met with some measure of success in alleviating the corrosion problem, it has been found that (at least when using an intermediate nickel layer beneath the subsequent layer of, for example, chrome or the like) if the zinc-coated aluminum article is permitted to remain for too long a period of time in the dissolving medium, some sort of passivation reaction appears to occur which interferes with the formation of a good metal to metal bond upon subsequent plating of the aluminum article.
It is therefore an object of this invention to provide a process for plating of aluminum using an intermediate zinc coating wherein the zinc coating is removed using a controlled removal process which provides an accurate monitoring of the removal rate and interrupts the removal of the zinc at a predetermined point.